The relation between body mass index (BMI; kg/m2) and longevity remains an area of interest, importance, and controversy. One point of controversy concerns the elevation in mortality risk at the low end of the BMI continuum. Most (though not all) studies observe a U- or J-shaped relation between BMI and mortality rate. A second controversy concerns the increase of the nadir of this curve with advancing age. Some authors have suggested that the elevated risk of mortality with low BMI and the increase in the nadir with age are artifacts due to putative confounding variables such as smoking and preexisting "occult" disease. However, even after attempts to control for these potential confounders, most studies continue to show U-shaped relations and increased nadirs with age. We (Allison et al, 1997) suggested an alternative explanation for these phenomena -- the use of BMI as a measure of "adiposity" may create artifactual relations. We propose to this issue at a new and more sophisticated level by studying the independent effects of measured fat and fat-free mass on mortality rate in humans by assembling a cohort of over 10,000 individuals on whom body composition measurements have already been obtained by one of four methods (dual energy x-ray absorptiometry; hydrodensitometry; isotope dilution; or potassium counting). These data will be obtained through investigators at multiple collaborating sites. Subjects will then be followed up with respect to mortality status. We aim to test the following prefatory and primary hypotheses: Prefatory Hypotheses. (1) There will be a U-shaped (or more generally convex) relation between BMI and mortality rate; (2) The convexity will not be entirely eliminated by controlling for age, sex, smoking, and apparent disease; (3) The nadir of the curve will increase with age. Primary Hypotheses. When body composition measures are simultaneously modeled: (1) The relation between fat free mass and mortality will be monotonic decreasing; (2) The relation between fat mass and mortality will be monotonic increasing; (3) The relations between fat mass and fat free mass and mortality will not change with age. To accomplish this, we propose an initial planning study in which complex statistical power analyses can be performed and inter-center coordination, subject follow-up/tracking mechanisms, and data management procedures can be established and pilot tested. The questions to be addressed have wide-ranging implications for clinical and public health recommendations for body weight management